Monitoring activity at multiple independent sites is often fundamental in understanding distributed processes underlying functioning of biological systems. The necessity to monitor the spatio-temporal activity patterns that are essential for information processing in neural systems are a primary example. The optical imaging is particularly fitted for this purpose. The goal of this proposal it to develop a system which would utilize optical imaging using voltage sensitive dyes and high frame-rate CCD camera in conjunction with multisite electrical stimulation to understand underlying mechanisms of neural communication and formation of spatio-temporal activity patterns in cortical cultures. Since the cultures form two-dimensional randomly connected networks, optical imaging will allow monitoring activity of every neuron in the field of view of the microscope, yielding information about neural communication with unprecedented detail. Application of optical imaging will allow observation of cell-cell interactions and formation of spatially distributed temporal patterns based on relative firing patterns of the interconnected neurons, whereas use of planar multielectrode arrays will enable stimulation of the network with stimuli having different spatio-temporal properties. Additionally we will develop analytical and numerical measures to monitor dynamical changes in cell interactions due to synaptic modifications. We will use them to test theoretically obtained prediction that relative excitation levels of interacting neurons can control relative spike timing and thus influence directionality of information flow in the network. In all, the developed system will allow us to monitor changes in patterns of activity of different neural types due to electrical and/or pharmacological manipulation, allowing for better future understanding of cell-cell interactions as well as providing a possible test bed for drug screening.